At present, two techniques of reflectometry are undergoing intensive research and development efforts, the two techniques being optical time domain reflectometry ("OTDR") and optical coherence domain reflectometry ("OCDR"). OTDR is an optical analog of radar and sonar. In accordance with OTDR, short light pulses having pulse durations in picoseconds, or even femtoseconds, are emitted from a suitable laser source and impinge upon a sample. The light pulses are reflected from various structures comprising the sample and the reflected pulses are detected by a time resolving detector. The distance between each reflecting surface of the sample and the detector is determined by reason of its proportionality to the time of flight of the light pulses from the light source to the reflecting surface and back again. In practice, the detection system can be a nonlinear, optical, cross correlation apparatus such as that described in an article entitled "Femtosecond Optical Ranging in Biological Systems" by J. G. Fujimoto et al., published in Optics Letters, Vol. 10, No. 3, March 1986, pp. 150-152. As described in the article, a beam of light reflected from the sample is superimposed upon a light pulse train emitted by the source in a nonlinear optical crystal. The optical pathlength of the reference beam is varied by moving a reference mirror which is mounted on a translation stage.
The resolution of the OTDR technique can be further improved by an optical coherence domain reflectometry ("OCDR") technique which is described, for example, in an article entitled "New Measurement System for Fault Location in Optical Waveguide Devices Based on an Interferometric Technique" by K. Takada et al., published in Applied Optics, Vol. 26, No. 9, May 1, 1987, pp. 1603-1606. In accordance with OCDR, a broad band, continuous wave source is used (in accordance with OTDR a pulsed light source is used). As shown in Section 7.5.8 of a book entitled "Principles of Optics," 6th Edition, M. Born and E. Wolf, Pergamon Press, New York (1986), the coherence length L of the broad band, continuous wave light source is related to its bandwidth B by the following equation: EQU L=c/B
where c is the velocity of light. In accordance with OCDR, output from the source is separated into two beams by a beamsplitter. One of the beams impinges upon a mirror which is referred to below as a reference mirror. The other one of the beams is directed to impinge upon a sample. Light reflected from the sample is superimposed with light reflected from the reference mirror. The superimposed beams interfere if the optical path difference between the two beams is smaller than the coherence length of the light source. Further, in accordance with OCDR, the reference mirror is moved with a constant velocity. As a result, the interference is detected as a periodic variation of a detector signal having a frequency equal to a Doppler shift frequency which is introduced by moving the reference mirror with the constant velocity. The interference signal vanishes as soon as the optical path difference between the beam reflected from the sample and the beam reflected from the reference mirror becomes larger than the coherence length of the light source. As those skilled in the art readily appreciate, displacement of the reference mirror must be in a range which corresponds to the depth of the sample to be imaged. Hence, OCDR is a technique which provides optical ranging with a high resolution, which resolution is limited only by the bandwidth of the light source.
OCDR is combined with a transverse scanning device to acquire three-dimensional images of semi-transparent objects such as the retina of the human eye in a technique which is referred to in the art as optical coherence tomography ("OCT"). This technique has been described, for example, in an article entitled "Optical Coherence Tomography" by Huang et al., published in Science, 254, Nov. 22, 1991, pp. 1178-1181.
In ophthalmoscopic applications of OCT, it is necessary to locate the field of interest on the fundus, i.e., the location where the retina is to be scanned by an OCT sample beam. FIG. 1 shows a figure from a Ph.D. thesis entitled "Optical Coherence Tomography" by David Huang, Massachusetts Institute of Technology, May, 1993 wherein a sample arm fiber of an OCT system is coupled to slitlamp biomicroscope 2000, a clinical instrument commonly used for examination of the eye. As shown in FIG. 1, a transverse scanning mechanism is mounted on slitlamp biomicroscope 2000 and two galvanometer driven motors allow the sample beam to be scanned in an arbitrary pattern on the retina. FIG. 1 shows OCT imaging device 2000 consisting of slitlamp viewing optics 2010 and ocular lens 2020 to image the fundus. As shown in FIG. 1, sample beam 2050, output from sample arm fiber 2060, is collimated by collimating lens 2070 and steered by orthogonally mounted, galvanometer driven mirrors 2030 and 2040. Focusing lens 2080 and dichroic mirror 2090 direct the sample beam into the image plane of slitlamp biomicroscope 2000. Then, ocular lens 2020, in combination with the optics of eye 2100, relays the image plane of slitlamp biomicroscope 2000 onto the retina. As disclosed, focusing lens 2080 and ocular lens 2020 form a telecentric system so that the sample beam which impinges upon galvanoscanner 2030 is imaged into the entrance pupil of eye 2100 and, as a result, vignetting is minimized. In addition, a red pilot beam is arranged to travel colinearly with the sample beam to enable an operator to see where the infrared sample beam is located on the fundus.
The ophthalmoscopic application of OCT shown in FIG. 1, and described in the thesis, suffers from several disadvantages. The first disadvantage results from the fact that the refractive error of a human eye varies within a range of up to .+-.20 diopters. Therefore, there is a need to focus the sample beam and the imaging optics of slitlamp biomicroscope 2000 to compensate for the refractive error of the human eye. However, in the apparatus shown in FIG. 1, focusing lens 2080 and slitlamp viewing optics 2010 are fixed. As a result, the required focusing is accomplished by moving ocular lens 2020 along the optical axis of slitlamp biomicroscope 2000. A disadvantage of this is that the image of galvanometer driven mirror 2030, as well as the image of slit illumination 2110, moves relative to the pupil of eye 2100 when ocular lens 2020 is adjusted. Thus, one must refocus illumination 2110 by moving microscope 2000. Then, one must refocus the image of the fundus, and so forth, to iterate to a position wherein both illumination 2110 and mirror 2030 are properly focused.
The second disadvantage results from adjustments which are typically made to overcome the effect of using a bright illumination light source in fundus imaging. It is necessary to use a bright illumination light source in fundus imaging because the low backscattering efficiency of the fundus (the fundus reflectivity is approximately 10-4) would otherwise result in a fundus image having a rather low light level. As shown in FIG. 1, and as described in the thesis, slit illumination 2110 is imaged into the eye pupil by ocular lens 2020. The reflectivity of the cornea of the eye and the reflectivity of ocular lens 2020 (in practice ocular lens 2020 is a Volk double aspheric bio lens manufactured by Volk of 7893 Enterprise Drive, Mentor, Ohio 44060) are both on the order of 4%, which reflectivities are much greater than that of the fundus. Therefore, it is necessary to make adjustments to keep backreflections from the cornea and from ocular lens 2020 out of the observation path of slit biomicroscope 2000. As disclosed in an instruction manual entitled "VOLK Double Aspheric Bio Lenses" published by Volk of Mentor, Ohio, at p. 3, adjustments are made to reduce backreflections by tilting slit illumination 2110 relative to the optical axis and by tilting ocular lens 2020. However, these adjustments cause astigmatism and vignetting.
In light of the above, there is a need in the art for method and apparatus for OCT fundus imaging which overcomes the above-described problems.